Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers.
One particular characteristic of a personal audio device which may affect its marketability and desirability is the dynamic range of its audio output signal. Stated simply, the dynamic range is the ratio between the largest and smallest values of the audio output signal. One way to increase dynamic range is to apply a high gain to the power amplifier. However, noise present in an audio output signal may be a generally monotonically increasing function of the gain of the amplifier, such that any increased dynamic range as a result of a high-gain amplifier may be offset by signal noise which may effectively mask lower-intensity audio signals.
Dynamic range enhancement (DRE) is a known technique to mitigate these issues. DRE is a three-stage process. In a first stage, digital gain is applied to an input digital signal; in a second stage, the digital signal is converted to the analogue domain by converter circuitry; and, in the third stage, an analogue gain is applied to the analogue signal. The digital gain may be determined dynamically, based on the amplitude of the input digital signal, and configured so as to increase the size of the digital signal at the input to the converter circuitry. In this way, the converter circuitry operates on a larger signal and as a result converts the signal to the analogue domain with lower noise. The analogue gain is configured to compensate for the digital gain, so that overall the signal is amplified to the required level, in spite of the dynamically changing digital gain. Thus DRE can be used to increase the dynamic range of an audio signal.
High quality audio playback is clearly a desirable feature for personal audio devices. However, such devices are becoming increasingly multi-functional, such that audio playback is only one of several functions which may be provided simultaneously by the device. For example, in a typical operating system there may be a variety of audio streams which can be classified into two groups: music (HiFi) and system sounds (keyclicks, alarms, ringtones). These different sounds must be mixed together into a single output audio stream.
Typically, such mixing is performed in software, and audio provided to amplifying circuitry as a single, pre-mixed audio stream.
FIG. 1 shows an example of this approach. Music 12 and system sounds, such as keyclicks 14, alarms 16 and ringtones 18 are generated by software running on a processor circuit (such as an applications processor) 10. Gain is applied separately to each audio signal in respective gain elements 20, and the outputs of each gain element are mixed in a combining element 22. The gains to be applied may be user-defined, or set in system configuration.
The mixed signal is provided from the processor circuit 10 to an amplification circuit, or codec, 24. In the illustration the processor circuit 10 and the codec 24 are provided on separate integrated circuits; however, in general the circuits may be provided on the same integrated circuit. In the codec 24, the mixed signal is converted to the analogue domain in a digital-to-analogue converter (DAC) 26, and provided to a power amplifier 28 which amplifies the analogue signal and outputs to an audio transducer such as a set of headphones or a speaker.
The disadvantages of this approach are that total harmonic distortion (or total harmonic distortion plus noise, THD+N) and dynamic range are limited by applying gain and mixing within the number of available bits of the digital signal. This both limits the dynamic range across all attenuation levels and increases distortion levels, limiting THD+N performance.